The long-term aim of this proposal is to understand the mechanisms that direct axons to their target fields in the trigeminal sensory system. In the last cycle of this grant, we studied and identified molecularly a chemotropic activity, Maxillary Factor, that was implicated in directing the gross pattern of trigeminal sensory axon projections to the periphery. Here, we propose to use single-cell cDNA differential screening technology and cDNA microarray technology to study the mechanisms responsible for the more discrete pathway choices made by subsets of trigeminal sensory neurons to form (1) the three trigeminal nerve branches, (2) the topographic projections to individual tactile hairs of the whisker pad, and (3) the divergent central terminations of large and small diameter sensory neurons. The approach will be to identify genes that are differentially expressed in subpopulations of trigeminal neurons projecting among distinct pathways, and to test the involvement in axon guidance of any of these genes that encode transmembrane proteins. The long-term aim is to define transmembrane receptors involved in guiding axons along different pathways, though the experiments should also reveal other molecular differences among the neurons, including in expression of transcription factors that regulated axon guidance receptor expression. This work will help define the molecular differences that regulate the distinct pathway choices of trigeminal sensory axons. In addition, it should be of more general importance in providing insight into the mechanisms through which discrete axonal pathway choices and topographic axonal projections are formed, two fundamental issues in axon guidance that are still poorly understood.